Every pulsatile blood pump has a volume change behind the diaphragm or pusher plate equal to that on the blood side of the diaphragm. It has not been a concern in the past since most blood pumps had been driven by external pneumatic drive systems or employed percutaneous vents. With the integration of implantable electrical and thermal drive systems with blood pumps, method to accommodate this variable volume must be developed. Prior studies by us as well as studies conducted in this research grant project have shown that a gas-filled intrathoracic compliance chamber is a simple and practical solution to the variable volume problem and demonstrated stable pressure-volume characteristics for over two-year durations in vivo. Sulfur hexafluoride, a non-toxic, medically used gas, appears to serve well as a buffer gas in the compliance gas mixture. Experiments currently underway are being used to quantify the exact composition of the compliance gas and the diffusional loss rate. A series of in vivo experiments have been started with sealed blood pump and compliance chamber combinations to evaluate overall system performance and to determine requirements for a gas make-up system. At the conclusion of the second year of this project, it is expected that the physical and operational characteristics of gas-filled, intrathoracic compliance chambers will be qualitatively and quantitatively determined. Since it is virtually impossible to eliminate gas diffusion through elastomeric blood pump and compliance chamber diaphragms, some means of gas addition is necessary for permanent clinical systems which are intended to be functional for a number of years. The specific aims are to: 1) develop a method to recharge the buffer gas which is diffusing out of the compliance space; 2) develop simple diagnostic method which can be used clinically to determine the gas volume of the compliance chamber; and, 3) demonstrate the overall performance of combined blood pump-compliance chamber-gas recharge systems in chronic in vivo experiments. At the conclusion of this project, the compliance subsystem will be ready for clinical trials as part of ventricular assist or total heart replacement systems.